A number of cellular communications systems are currently being introduced. In Europe an operating standard known as GSM has been set up to define protocols, services and operating equipment.
The GSM Standard has been established as a means of realising a pan-European digital wireless telephony network, operating according to the set of ETSI specified protocols known as the GSM recommendations. The primary purposes of these recommendations is to provide a wireless telephony service to subscribers who are free to roam and use the same mobile equipment and one reachable by the same telephone number anywhere within the operating area. This may comprise a number of different networks within the same country, and even in a number of different countries, while subscribers receive a base level of service independent of their location. The recommendations define a GSM network to consist of a number of different types of functional elements, which may be variously combined in an indeterminate number to form a Public Land Mobile Network (PLMN). Within a given country there may be more than one PLMN, and a number of countries are signatory to a memorandum of understanding (the GSM MoU) which requires them to license and make available GSM telephony service which is interoperable with the GSM telephony services provided by the other signatory countries.
The principle telephony components of the GSM network are the home location register (HLR), mobile services switching centre (MSC), the visitor location register (VLR), the base-station controller (BSC), base transceiver stations (BTS), and finally the mobile equipment (ME) plus a subscriber identity module (SIM) which together define a mobile Network (PSTN) and to other PLMNs through an interface function known as the gateway MSC (GMSC). Other functional components exist to provide administration and maintenance of the network. While individual components are allowably realised in combination, (examples being the combination of VLR and MSC functionality in the same network element, or the MSC and GMSC functionality in the same network element) in usual practice a plurality of network elements are deployed in geographically separated locations in order to provide effective physical radio coverage of the network territory. Communication between network elements is usually accomplished by a known messaging protocol based on the CCS No 7 signalling system.
Each country signatory to the GSM standard has licensed (or has plans to do so) one or more PLMNs. Each of these PLMNs is separately licensed and regulated according to the constitutional and national laws relating to telecommunications within that country. Due to commercial and competitive pressures, it is usual for PLMNs within the same country to offer different services and functionality in order to attract and keep the loyalty of their customer base (mobile subscribers). It is also competitively advantageous for PLMNs to secure the widest range of interworking agreements between other PLMNs, especially in other countries, thereby allowing their own subscribers access to a high degree of mobility.
Due to competition within the telecommunications industry, there are multiple vendors for each of the various network elements. These vendors differentiate themselves not only on the basis of territorial presence, reputation and price, but also on the basis of differential functionality. As a consequence, differentiation between PLMNs within the same country is in many cases accomplished through using equipment procured from different manufacturers. It is also usual practice for a given network operator to procure equipment from more than one supplier, in order to maximise competitive pressures upon the suppliers by avoiding monopoly supply situations. As a consequence of the combined pressure of multiple supply situations, and the desire to maximise roaming potential, interworking between equipment manufactured by various vendors across various legal, geographic and national boundaries is imperative. Roaming from one network into another is described for example in specification U.S. Pat. No. 5,329,574. A discription of a method of managing supplementary services in a mobile system is given in specification No WO-A1-94/10814. Specification NL9301493A relates to verification of access rights in a mobile network.
Several fundamental problems are inherent in such a distributed multi-vendor multi-operator architecture, which the introduction of the GSM recommendations as a "standard" is intended to resolve. However, adherence only to standardised implementation by all manufacturers is in direct conflict with competitive pressures, and in some cases with regulatory requirements. As a consequence, various differential realisations of GSM functionality above and beyond the basic standards known as GSM are a fact of the business, and introduce several issues related to operations, administration, maintenance and procedures which have commercial impact not only towards cost of ownership, but also towards manufacturability.
Although the GSM recommendations describe comprehensive functionality, they permit differential implementations in recognition of the need for competitive differentiation between manufacturers: it is common in practice for manufacturers to differentiate their products through the addition of proprietary extensions, the facility for which is included in the inter-element message protocols. Given the nature of proprietary extensions and that they arise as a consequence of commercial pressure, it is likely that different manufacturers will independently implement similar functionality for different customer's and subsequently leverage this additional functionality as commercially advantageous to other customers. It is highly unlikely that the different manufacturers will use identical message protocols. Since network operators commonly procure equipment from more than one supplier, they require functionality between manufacturers' equipment to be available to their mobile subscribers as conditions of supply. Even without dual suppliers within the same network, inter-PLMN roaming agreements impose similar interoperability pressures on manufacturers. Finally, to maximise competitive advantage within a country, the same manufacturer may be required to supply functionality to one PLMN not available to the other PLMN. As a consequence of these commercial pressures, it is highly likely that two different manufacturers would be frequently in the position of having to supply similar functionality with differential implementations, which implementations must interwork together, and simultaneously supply differential functionality which must be prevented from interworking with that of their own or other manufacturers' equipment. The manufacturers must therefore find a mechanism to control interworking of differential functionality between each other's equipment such that unexpected interactions do not occur. Since it is also highly likely that independent manufacturers introduce proprietary functionality independently at different times, a flexible means of controlling differential functionality is of joint commercial advantage to both manufacturers (they can introduce equipment that might not otherwise behave in a deterministic manner) and network operators (they can control appearance of differential functionality within the network).
The various geographically distributed network elements must work together to provide each instance of a mobile subscriber's service. For example, an incoming call to a mobile subscriber requires functionality resident in the MSC, VLR, HLR, BSC, BTS, ME and MS at various stages of establishing the call. Delivering service in this environment implies that all nodes must be able to interact with their neighbours, which neighbours may be in different PLMNs and indeed in different countries. However, the service set is continuously being revised, and inconsistencies between the various network elements are unavoidable or, when avoidable, impose significant operational complexity on the network operators and equipment manufacturers alike. There is a real need to maintain compatibility between network elements firstly between networks, but also within the same network, even between equipment provided by the same manufacturer but which equipment may be at different functional release levels. It is practically impossible and wholly undesirable to simultaneously upgrade all of the network elements in all of the networks in the world simultaneously. It must therefore be recognised that transient situations will continuously appear during which differential services may exist in the network as a consequence of network upgrades occurring world-wide, exacerbated by the presence of inter-PLMN roaming agreements. It is commercially desirable that a network operator can "roll out" new services throughout the network as they become available, rather than have to wait for the network to be fully upgraded, as this increases the velocity by which new revenue generating services can be made available to subscribers. It is therefore advantageous to be able to permit differential exchange of messages between two network elements depending on their ability to interpret the information. Similarly, there is an issue of compatibility between various national networks, where regulatory or legal constraints may restrict identically implemented and otherwise "standard" services such as calling line presentation. Thus, services normally available to a given MS, and known and available as part of the functionality which could be deployed, must be restricted from use.
In addition, it is commercially advantageous for a network operator to offer differential grades of service, perhaps at premium prices, which restrict services which are otherwise semantically and syntactically embedded in the fundamental service definition. The simplest example of this in GSM is the ability to offer a "basic" roaming service to customers who would be restricted from operating their mobile phones outside the home PLMM, and offering increasing roaming capability as a premium service for additional revenue.
The object of the invention is to minimise or to overcome the above disadvantages.